Gravity surveying is one technique in modern exploration for mineral and petroleum commodities. For example, detection of geophysically significant subsurface anomalies potentially associated with ore bodies or hydrocarbon deposits can be made using gravity surveying techniques since the existence of gravitational anomalies usually depends upon the presence of a change in mass density associated with the deposit. For example, the gravitational anomaly of a body of ore with a density contrast of 300 kg m−3 and a dimension of 200 m buried at a depth of 100 m is typically 2×10−6 m/s2, for example, which is 0.00002% of the normal Earth gravity field. This relatively small effect is typically measured in units of milligals (mGal), which is the unit for the free air and Bouguer gravity field measurements and is equivalent to 10−5 m/s2. Thus, for the above example, the body of ore would be represented by 0.2 mGal.
Currently, many measurements have been made using instruments of the LaCoste/Romberg type that are essentially a mass (e.g., proof mass) balanced on an ultrasensitive spring that detects a difference in apparent weight caused by the change in gravity. The measurements are subject to a wide variety of environmental influences, and measurements should be performed relative to a standard point that is used regularly during the survey as a fixed reference for removal of drifts in the instrument. This procedure can be slow, and may require extensive information on local topography and geology since a normal variation of gravity with height is approximately 0.3 mGal per meter. Within moving platforms, such as aircraft, using this type of relative gravity instrument can be difficult because using precision radar altimeters and pressure sensors to achieve vertical position to as little as one meter can impose limitations on the order of a few hundred mGals on the gravity data. Also, accelerations caused by turbulence and normal flight control can cause apparent gravity changes many times larger than the measured gravity variations.
For this reason, some geophysical prospecting has progressed towards gradiometry. In principle, measurement of a gradient of a gravity field over a known baseline allows accelerations due to motion of the platform itself to be cancelled out. Gravity gradients are the spatial derivative of the gravity field, and have units of mGal over distance such as mGal/m. The standard unit of gravity gradiometry is the Eötvös (E), which is equal to 0.1 mGal/kilometer or 10−9/s2 (e.g., gradient signatures of shallow Texas salt domes are typically 50 to 100 E).
Further development has led to the progression toward three-dimensional Full Tensor Gradient (3D FTG) technology, which was developed by the US Navy and later adapted to the oil and gas industry to complement seismic technology and provide an independent method of imaging around salt and basalt areas, for example. Full tensor gradiometry measures the X, Y, and Z Cartesian components of the gradient of the gravity field.
Acquisition of geophysical data requires an operator to consider the unique nature of such a high frequency, small amplitude measurement. Acquisition parameters are dictated by several factors including, for example, water depth, target depth, terrain density, geologic concerns and the type of imaging problem being modeled.
Geophysical data includes readings from a variety of sensors that are within the survey vessel. Different types of sensors include altimeters, gravimeters, electromagnetic sensors, and magnetometers, for example. At times, it may be necessary to relate or synchronize readings from one or more of the sensors. Thus, it would be desirable to receive data from one or more additional sensors within the survey vessel and to identify when and/or where the data was collected to be able to associate data recorded from one sensor with data recorded from one or more sensors.